A group of Senate Democrats is threatening to filibuster a controversial gun rights amendment to the defense authorization bill, saying it goes too far.

At issue is an amendment offered by Republican Policy Committee Chairman John Thune (S.D.) that would allow gun owners to carry concealed firearms into states with similar gun regulations.

Democratic opponents are working to rally at least 41 Senators to vote against the measure. Those foes lack the support of many in the moderate wing of the Conference, including ailing Sens. Robert Byrd (W.Va.) and Edward Kennedy (Mass.) and Sen. Mark Begich (D-Alaska), who is co-sponsor of the amendment.

“This amendment is a bridge too far, and could endanger the safety of millions of Americans,— Democratic Conference Vice Chairman Charles Schumer (N.Y.) said in a statement. “We will do everything we can to stop this poisonous amendment from being enacted.—

Democrats are closely counting votes for the measure, which will be considered on the floor as soon as Wednesday. A coalition of 400 city mayors is assisting with the counteroffensive by lobbying Congress, and Democratic Sens. Frank Lautenberg (N.J.), Bob Menendez (N.J.), Dianne Feinstein (Calif.) and Schumer, along with New Jersey Gov. Jon Corzine (D), will speak against the amendment Tuesday.

Democrats were rattled earlier this year when an amendment sponsored by Sen. Tom Coburn (R-Okla.) allowing guns on federal lands usurped legislation granting the District of Columbia a voting Member of the House. Coburn’s amendment has all but killed the bill.